Implantable medical devices that provide therapy for conditions such as tachycardia and/or cardiac fibrillation must create a high voltage charge within a storage element such as a bank of capacitors. The capacitors then provide the energy for the tachycardia or defibrillation therapy. To create the high voltage charge, a battery is switched to produce pulses that are provided to a transformer that steps up the voltage.
The implantable medical device may encounter ambient conditions that can impair the transformer by saturating the transformer core. A magnetic resonance imaging (MRI) scan is one example of a situation where the ambient conditions include a large static magnetic field that may saturate the transformer core. While devices may deactivate sensing and therapy by entering a special mode designed for MRI scans, there may still be a risk that the device inadvertently exits the special mode prematurely during the MRI scan due to a device reset caused by the strong fields of the MRI machine or due to some other unforeseen reason. Alternatively, there may be a risk that the device is inadvertently not programmed into the special mode designed for MRI scans prior to the patient receiving the scan.
When the strong magnetic field of the MRI machine or other ambient condition saturates the core of the transformer, the inductance of the primary coil is drastically reduced. The primary coil then presents a very low impedance to the battery which causes the current from the battery to increase and the voltage to decrease. This spike in current may damage components of the implantable medical device including the battery.